1. Field of the Invention
The present invention relates generally to blast nozzles and a process for removing adherent material such as paint, scale, dirt, grease and the like from solid surfaces with abrasive particles propelled by air. In particular, the present invention is directed to a novel blast nozzle having an improved water atomizer means useful to control the dust caused by blasting with an abrasive and friable media such as sand or sodium bicarbonate.
2. Description of the Prior Art
In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, or simply to degrease a solid surface such as surfaces contacting food or building structures which contain food serving or food processing operations, it has become common practice to use an abrasive blasting technique. In such process, abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove the coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation.
A typical dry blasting apparatus as well as a wet blasting apparatus which utilizes highly pressurized air to entrain, carry and direct the abrasive blast media to the solid surface to be treated and low pressure water for dust control comprises a dispensing portion in which the blast media typically contained in a storage tank is entrained in highly pressurized air, a flexible hose which carries the air/blast media mixture to the blast nozzle and which allows the operator to move the blast nozzle relative to the surface to be cleaned and the blast nozzle which accelerates the abrasive blast media and directs same into contact with the surface to be treated. Water is added either internally in the blast nozzle and mixed with the air stream passing therethrough or a low pressure stream of water is provided externally of the blast nozzle and directed at the surface to be treated so as to control dust. The blast nozzle is typically hand-held by the operator and moved relative to the targeted surface so as to direct the abrasive blast media across the entire surface to be treated.
The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to non-soluble blast media such as sand, in particular, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. Unfortunately, sodium bicarbonate, typically used as particles having average diameters of from about 50 to 1,000 microns, is even more friable than sand and breaks into smaller particles as it traverses the flexible supply hose which carries the blast media and pressurized air to the blast nozzle and, as well, breaks into pieces as the blast media comes into contact with the internal surfaces of the blast nozzle prior to being propelled to the target surface. As the sodium bicarbonate media contacts the surface to be treated, even smaller particles are formed yielding a substantial amount of dust which invades the targeted area and closely surrounding environment, hindering the operator's vision of the targeted surface. Accordingly, it has become necessary to control the dust which is formed upon blasting with the very friable sodium bicarbonate blast media.
As expressed above, it is possible to control dust by injecting a low pressure stream of water into the air stream which propels the blast media. This has been accomplished by two distinct methods. In one method, the blast nozzle is provided with a water port in which water is injected into the blast nozzle to mix with the air stream and entrained blast media particles. This method has been very effective in controlling the dust of the sodium bicarbonate particles subsequent to contacting the targeted surface. Unfortunately, in view of the low density of the sodium bicarbonate particles and the water solubility thereof, the velocity of the media particles is reduced by the water and consequently, the productivity with respect to cleaning the targeted surface is substantially decreased by this method. Thus, defining performance of a blast nozzle as the rate in which a volume of coating is removed per time, injecting the water with the air stream which propels the blast media has greatly reduced the production rate for the reasons expressed above.
An alternative method has been to direct the low pressure water stream externally from the blast nozzle at the targeted surface to control the dust which forms at the contact point. While this process has yielded improved productivity relative to the internally directed water stream, dust control is only slightly improved relative to dry blasting and substantially inferior to the process in which the water stream is directed internally in the blast nozzle. In view of the advantages of utilizing sodium bicarbonate as a blast media as enumerated above, including water solubility to improve clean up, less harmful to the environment and useful to clean a wide variety of different surface types, it certainly would be most advantageous to improve the processes and apparatus for using same. In particular, it would be most advantageous to reduce the dust associated with the sodium bicarbonate blast media and, at the same time, maintain the productivity found using sodium bicarbonate as a blast media in dry blasting.
An improved blast nozzle for directing an abrasive media against a targeted surface in a pressurized air stream for the removal of surface coatings, scale, dirt, grease, etc. and to reduce dust formation is described in copending, commonly assigned U.S. Ser. No. 07/958,552, filed Oct. 8, 1992. As disclosed therein, the blast nozzle is provided with an external source of atomized water which is also directed at the targeted surface so as to control the formation of dust. The atomized water is achieved by an atomization nozzle in which air and water are mixed and directed from the nozzle in drops having a diameter of about 15 to 100 microns. The atomized water is directed at the targeted surface at a location to meet deflected abrasive media particles which have contacted the targeted surface and coalesces or otherwise precipitates the minute particles of blast media, thus greatly reducing the dust which is formed. At the same time, the minute atomized water particles provided at low pressure and externally from the blast nozzle do not substantially interfere with the media flow from the blast nozzle to the targeted surface and, thus, maximum velocity of the blast media is substantially maintained and productivity for stripping or cleaning the targeted surface is maintained at high levels, approaching those levels achieved for purely dry blasting operations.
The nozzle of the above-described application is substantially more effective in controlling dust and maintaining the productivity of the nozzle than previous blast nozzles which used other dust control techniques. However, it was found that with one water atomization nozzle mounted on the blast nozzle, dust control can still be improved. Additionally, the water atomization nozzle was disclosed as being mounted adjacent to the outlet of the blast nozzle. Consequently, the atomized water droplets issuing from the water atomization nozzle were not in contact with the dust cloud long enough to coalesce the dust particles before the water droplets contacted the targeted surface.
Accordingly, an object of the present invention is to provide a blast nozzle which can provide good dust control when utilizing a friable blast media to clean a targeted surface.
Another object of the present invention is to provide an improved blast nozzle which is useful in directing an abrasive but friable blast media against a targeted surface for the cleaning thereof without yielding excessive dust and, at the same time, maintaining the productivity of the nozzle at high levels.
Still another object of the present invention is to provide a blast nozzle useful in directing sodium bicarbonate in a stream of air against a targeted surface for the cleaning thereof and capable of controlling the dust which results when the sodium bicarbonate blast media contacts the targeted surface.